docs: Move from sphinx to Mkdocs

[alhendrickson]

See here for the results:

The nifi Readthedocs page:
https://cogstack-platform-toolkit--89.org.readthedocs.build/projects/nifi/en/89/

And then the integrated docs page:
https://cogstack-platform-toolkit--122.org.readthedocs.build/en/122/data-engineering/nifi/

Notes

Generally tried to keep this as small as possible and not change the actual contents of the docs. It should generally look like it does today with a different theme.

Later would probably want to reorganise to make the most of the nav sections / headers that mkdocs can do. I've tried to keep it exactly as is today for now.

All the setup / config etc should be the same as in the medcat and medcat-trainer docs pages

index.md

docs/index.md is a straight copy+paste of the README.md. The snippet used before isn't going to work well later (see change in cogstack-nlp). Long term would want README.md to change to being content relevant for someone who has cloned it (like it is now), but index.md be relevant to someone viewing the docs page. For example showing the directory structure on the docs page isn't really needed.

attributes

.gitattributes change is to fix these coming up as modified on a new clone. These zip files probably should get deleted though as I see them listed .gitignore as well.

        modified:   security/certificates/elastic/elasticsearch/es_native_certs_bundle.zip
        modified:   security/certificates/elastic/elasticsearch/es_native_certs_bundle_pem.zip

[vladd-bit]

lgtm